Two rules: Wear a helmet and don't be a dick. Actually I may be thinking of something else. Never mind.

There are well appointed camp grounds in both Arapilies and the Grampians. That is: no walk in, the toilets either flush or are sufficiently deep, and there is plenty of flat ground. Depending where you want to climb in the Grampians these may not be convenient.

Take water, you don't need it every where, but life is much easier if you have it. Ditto for bog roll.

If you don't have either a partner or a guide book, go to Arapilies.

There is a notice board at the toilet block in the Pines (Arapilies), check there for info on access issues, lost and found gear, free sex, etc.

Camping at Arapiles is allowed only in the campground.
The Grampians is a National Park and there's lots of rules about where you can and cannot camp. Get a park map and check out the Parks website for camping do's and don'ts.
You're probably going to Stapylton at some point so you need to know that bush camping is not permitted close to Stapylton. The Stapylton campground is your closest spot.
Both Arapiles and Stapylton get lots of visitors so don't trash the bush by making your own campsite.